Experimental analyses of the function of bird song have indicated that even in extremely diverse songsters, those features serving species recognition remain relatively constant from individual to individual. Mockingbird song is unusually diverse perhaps because, as natural mimicks, Mockingbirds, can incorporate the sounds of other species into their songs. Mockingbirds, moreover, sing during both the Spring and the Fall, and species recognition is presumed to be among the functions served by song in each season. The results of earlier experiments indicate, however, that the Sping and Fall songs of this species differ substantially along a number of dimensions, and that the song normally sung in the Spring is ineffective in eliciting species recognition in the Fall. The series of experiments proposed here will employ spectrographic analysis and the playback of artificial songs in an attempt to demonstrate the importance of Fall song for species recognition in the Fall. Additional studies will attempt to differentiate those features responsible for species recognition in each season. Seasonal separation of function in particular song features implies unusual flexibility in song. The results of these analyses may provide a foundation for understanding the evoluntionary bases for both the diversity and the flexibility of Mockingbird song.